In general, in combustion furnaces (combustion systems) typified by industrial furnaces such as a steel furnace, a heating furnace and a deodorizing furnace, a combustion control is performed by a combustion controlling device while monitoring a combustion state of a burner disposed in the combustion furnace, a furnace temperature, a pressure of a combustion air, and a pressure of a fuel to be supplied to the burner, to thereby ensure safe combustion. For example, in order to prevent the explosion of the combustion furnace, the combustion controlling device executes an ignition of the burner after performing purge (prepurge) for discharging a residual fuel (gas) in the combustion furnace to an outside of the combustion furnace at the time of igniting the burner, determines whether the burner is ignited, or not, with the use of a flame detector, and performs a safety control for stopping the supply of the fuel to the combustion furnace when the burner is not ignited (for example, refer to Patent Document 1).